For electrical safety, to allow operation with large common mode signals or for both certain types of circuits require electrical isolation between two sub-circuits. One way of implementing an isolation barrier between sub-circuits is through the use of an optical isolator or optoisolator. Another way is through the use of a transformer. These circuits often also require a signal path between the two sub-circuits.
The signal path is often implemented using a conventional optoisolator, since it is usually cheaper than a transformer. FIG. 1 is a simplified circuit diagram illustrating an example of a conventional optoisolator signal path. In FIG. 1, circuits 10 and 20 are electrically isolated from one another. However, optoisolator circuit 30 provides a signal path from transmitting circuit 10 to receiving circuit 20. A signal output from transmitting circuit 10 drives light-emitting diode (LED) 32, which emits light that is received by the base of photo-transistor 34. When LED 32 is active, photo-transistor 34 will conduct current into the base of transistor 36, which amplifies the received current signal. The received signal may then be sensed by receiving circuit 20. Typically, optoisolator 30 is a standard external part that is relatively expensive.
FIG. 2 is a cross-sectional view of an example of a conventional photodiode circuit 50 that includes a reflective dome 52 in the plastic packaging 54 to the circuit. The dome is molded into the package, which is typically an expensive process, and a reflective or white coating is formed over the packaging to improve the reflectance. The dome and reflective coating are utilized to improve the current transfer ratio (CTR), i.e. the ratio of the current required to drive the LED 56 versus the current generated in the receiving photodiode 58, of the optoisolator circuit. In addition to the higher cost associated with molding the dome structure, the dome structure can cause difficulties in fitting the optoisolator package into its ultimate application.